The invention relates to a discharge prevention circuit, electronic equipment provided with the discharge prevention circuit, and, in particular, to a discharge prevention circuit for preventing a capacitor from being discharged. The invention may be used in a system where a DC power supply voltage is supplied from a power feed line to electronic equipment having the capacitor for storing energy at the power input, and similar electronic equipment may be sequentially and additionally connected to or disconnected from the power feed line.
As the demand for cellular phones and connection with the Internet have increased in recent years, communication companies such as Internet service providers, and telephone communication companies continue to install add-on units of communication equipment. And the number of add-on units is increasing. In general, DC power is supplied from a power unit to the respective units of communication equipment via a power feed line. In this case, the number of units of the communication equipment connectable to one power unit is limited by the power supply capacity of the power unit and the power consumption of the respective units of communication equipment. In general, a large-capacitance capacitor for storing energy is often incorporated at the power input portion of each unit of communication equipment. When an add-on unit of communication equipment is sequentially and additionally connected to a power feed line while a DC power supply voltage is supplied from the power feed line to already installed units of the communication equipment, a capacitor incorporated in the power input portion of the add-on unit of the communication equipment is charged, and then, the DC power supply voltage of the power feed line momentarily drops, thereby causing at times a problem in that the already installed units of the communication equipment stop operation, resulting in stoppage of operation of communication service. As a countermeasure for this problem, there is available a discharge prevention circuit as disclosed in Japanese Laid-open Patent No. 315201/2002 (in particular, page 1 to 2, drawings: FIGS. 1 and 4)
FIG. 1 is a circuit diagram showing an electrical configuration of the principal part of electronic equipment provided with a discharge prevention circuit disclosed in Japanese Laid-open Patent No. 315201/2002.
The electronic equipment is communication equipment 10, connected to a DC power unit 1 via a power feed line 2, having a capacitor 11 installed in a power input portion. A DC power supply voltage Vin from the DC power unit 1 is applied to the capacitor 11 and charge the capacitor 11 so that the voltage of the capacitor 11 is substantially the same voltage as the DC power supply. The capacitor 11 also supplies power to a load 12. The load 12 may be internal circuits such as transmitters, receivers and processors of the communication equipment 10. Further, the communication equipment 10 is provided with a diode 13 so that a discharge prevention circuit is realized. In the case that the voltage of the DC power supply Vin momentarily drops, for example, by connecting additional communication equipment to the power feed line 2, the diode 13 prevents reverse current flow from the capacitor 11 to power feed line 2.
The communication equipment 10 has a simple configuration, as shown in FIG. 1. In the communication equipment 10, the diode 13, which is a passive element, is used as the discharge prevention circuit. Therefore, a complicated control circuit is not necessary in this configuration. However, with this discharge prevention circuit, assuming that a load current is Io and a forward voltage of the diode 13 is Vf, electric power of Io×Vf is always consumed. For this reason, the diode 13 needs to have a configuration comprising a plurality of diodes connected in parallel and needs to have effective heat dissipation capacity such as a large-size heat sink, therefore the volume of the configurations has to be large. Accordingly, it is difficult to provide equipment consuming a lot of power with the discharge prevention circuit disclosed in Japanese Laid-open patent No. 315201/2002.
FIG. 2 is a circuit diagram showing an electrical configuration of the principal part of electronic equipment provided with another discharge prevention circuit disclosed in Japanese Laid-open Patent No. 315201/2002. In FIG. 2, elements in common with those in FIG. 1 are denoted by similar reference numerals.
The electronic equipment is communication equipment 10A, connected to a DC power unit 1 via a power feed line 2. With the communication equipment 10A, there is provided a discharge prevention circuit comprising a current transformer 14, a control circuit 15, and an n-channel enhancement MOSFET 16 in place of the diode 13 in FIG. 1.
In the communication equipment 10A, the MOSFET 16 is in the ON state during steady state (at times except with the voltage of the capacitor 11 is lower than the normal DC power supply voltage due to a temporary drop in the DC power supply voltage), and while a DC power supply voltage Vin from the DC power unit 1 is applied to a capacitor 11 via the current transformer 14 to thereby charge the capacitor 11, power is supplied to a load 12. When the DC power supply voltage Vin momentarily drops upon connection of equipment similar to the communication equipment 10 to the power feed line 2, reverse current begins to flow from the capacitor 11 to the power feed line 2. However, this current from the capacitor 11 is then detected by the current transformer 14, and the current transformer 14 outputs a reverse-flow detection signal. The reverse-flow detection signal a is inputted to the control circuit 15, and a control signal b is outputted from the control circuit 15. The MOSFET 16 is turned into the OFF state by the control signal b, and then the reverse flow of the current from the capacitor 11 to the power feed line 2 is prevented, thereby preventing the capacitor 11 from being discharged. Specifically, when the voltage between source and gate of the MOSFET 16 is high level, the MOSFET 16 is the ON state. On the other hand, the voltage between source and gate of the MOSFET 16 is low level, the MOSFET 16 is the OFF state. Further, at the time of the steady state, there occurs a loss due to on-resistance of the MOSFET 16, but heat generation is significantly less in comparison with the case of the discharge prevention circuit using the diode 13 shown in FIG. 1.
However, the related discharge prevention circuits described as above have the following problems. In the discharge prevention circuit of FIG. 2, the large current in the steady state and the momentary changing current have to be taken into account for circuit designing, because the current transformer 14 is connected in the main current line (between the DC power unit 1 and the load 12). Furthermore, power consumption happens at the current transformer 14, because a large DC current (as well as a momentary changing current) constantly flows through the current transformer 14 due to the configuration in which a large scale current transformer 14 is disposed between the power feed line 2 and the load 12.